1. Field of the Invention
The invention relates generally to electronic oscillators, and more particularly to a fully differential voltage controlled oscillator.
2. Description of Related Art
Without limiting the scope of the invention, this background information is provided in the context of a specific problem to which the invention has application. Voltage controlled oscillators a.k.a. VCOs are ubiquitous, found in applications ranging from automobiles to microprocessors. The most common application for a VCO is in a phase locked loop, a.k.a. PLL. A PLL is commonly used to synthesize a high frequency clock signal from and synchronous to, a lower frequency reference signal.
In its most simplest form, a PLL comprises a VCO, a prescaler (divider), and a phase comparator (sometimes referred to as a frequency/phase discriminator). The output of the phase comparator drives the input of the VCO, typically through a low pass filter. The VCO output is then scaled through the prescaler and fed back to one input on the phase comparator. A second input on the phase comparator is coupled to a reference signal that the VCO output frequency tracks, scaled by an amount set by the prescaler.
In digital systems, power supply noise can have a dramatic effect on accuracy. For the most part, noise superimposed on the power supply is from the fast switching digital signals. This noise tends to be common mode thus is present on both rafts of the power supply. Accordingly, differential devices are typically used to cancel the unwanted common mode signals. Up until now however, digital systems employing VCOs have been either single-ended or did not truly have differential inputs and outputs operating substantially 180 degrees out of phase. Consequently, the common mode rejection ratio was marginal. Furthermore, prior art VCO approaches are not particularly adept at operating at reduced power supply voltages.
By way of illustration, U.S. Pat. No. 4,692,717, issued Sep., 8, 1987, entitled "Voltage Controlled Oscillator With High Speed Current Switching" discloses a single-ended VCO having a single capacitor (10) for filtering noise on the V.sub.cc power supply rail but not on the ground rail. Moreover, an active filter is employed for the loop filter making it difficult to operate at a reduced power supply voltage.
Similarly, U.S. Pat. No. 4,812,784, issued Mar. 14, 1989, entitled "Temperature Stable Voltage Controlled Oscillator With Super Linear Wide Frequency Range" discloses a VCO including capacitors C.sub.1a, and C.sub.1b, which provides, for purposes of power supply noise rejection, undesirable feedback into nodes A and B. Additionally, the design requires the so-called BiCMOS fabrication process, a combination of bipolar and CMOS technologies having its own drawbacks.
Accordingly, it can be seen that there is a need for a fully differential VCO capable of operating at a reduced power supply voltage and having a large common mode rejection ratio (CMF) that provides a superior power supply rejection ratio (PSRR).